"That's too much echo... echo... echo... Turn it off... off.. off..." --
Stan Freberg, "Heartbreak Hotel".

Twenty years ago, JVC and Yamaha made consumer reverb units whose programs
were modeled after specific churches, concert halls, and other performance
venues. If you're trying to produce a natural sense of reverbererberation,
this sort of device is what you want.

You should be looking for a Yamaha DSP-1, DSP-3000, JVC XP-A1000, XP-A1010.
I don't remember if the DSP-1 has a digital input; the other models do. The
DSP-1 requires its remote control and is useless without it. The others can
be operated from their front panels but it's a bit clumsy and inconvenient
to do so.

All offer four outputs, two rear and two side. The programs are adjustable,
to match the sound of the synthesized reverb to the ambience of the
recording.

They sometimes show up on eBay. The Yamaha DSP-1 is fairly common, the
others less so. I recently bought a JVC XP-A1010 as a backup to the XP-A1000
I already own. (I also have a Yamaha DSP-3000 and Lexicon CP-3plus.)

You should always run the ambience through added speakers. You should
_never_ mix it with the original. It screws up the sound quite badly.